Heretofore, various circuits have been proposed for the measuring current flowing in an A.C. distribution line.
Perhaps, the simplest current sensing apparatus is a current transformer. Such a transformer has at least one turn of a line conductor forming the primary winding or the input, a multi-turn secondary winding which develops a current directly related to the line current and a iron core to magnetically link the primary and secondary windings. The device is simple, although it does have some disadvantages. For example, there exists the possibility that the iron core may exhibit residual magnetism after the occurrence of high fault current. A more fundamental disadvantage is that such a transformer cannot be safely used because the output leads of the secondary winding are normally used at ground potential and the primary winding is normally the high voltage line conductor. To be safely used, expensive and bulky electrical insulation must be used to separate the current transforming high voltage primary winding from the ground potential used secondary winding.
U.S. Pat. No. 4,384,289 discloses a device for measuring current on live transmission lines. That device utilizes a toroidal shaped transponder which includes a hub and concentric spoke mounting arrangement that is connected to an energized power transmission line by means of a spring-loaded hinge mechanism. It includes a radio frequency transmission circuit and an antenna so that the current line can be monitored remotely.
U.S. Pat. No. 4,471,355 provides an apparatus for sensing and producing an amplitude and phase replica of an alternating current signal using a toroidal coil and a fiber optic transmission line. Unfortunately, it requires the use of a bulky and complex power supply which is capacitively connected to the high voltage conductor and it fails to provide a convenient and directly usable source of digital signals.
U.S. Pat. No. 4,630,218 also uses a fiber optic cable and a current measuring apparatus. It uses a current sensing and pulse transmitting apparatus that is directly powered from a current transformer. It is basically a non-linear device in the the CT drives a resistor until one zener diode conducts and then on the next half-cycle another zener diode operates at a different voltage.
A more complicated current measuring apparatus is disclosed in U.S. Pat. No. 4,642,564. This patent describes a device which may be used to monitor a three-phase distribution system. No teaching is provided with regard to digital encoding and decoding for use by a microprocessor.
Finally, there is the apparatus taught by U.S. Pat. No. 4,625,283. That device makes use of a current transformer and an electronic circuit which senses the crossing of a negative reference voltage and a positive reference voltage of the same magnitude. Unfortunately, there is no teaching with regard to electrical isolation and the use of the device in a field environment. More importantly, it, like the device of the '218 patent, does not have high resolution when high currents are encountered.
Thus, while the art recognizes the benefits of electrical isolation between the line monitored and the monitoring apparatus, the use of a current transformer to provide a current replica of the line to be monitored, the use of microprocessors and associated electronics to provide a computation of the line current based on pulsed information, it has failed to provide an apparatus which can be used utility personnel to monitor the line current at a distribution feeder line. More importantly, the art does not teach an apparatus which can be quickly installed by field personnel without requiring deenergization of the monitored line, or direct electrical contact with the monitored line, and an apparatus which has high resolution when high currents are measured. Clearly, despite the number of recent developments in the art, there remains room for improvement.